In order to obtain a brighter and clearer image, a complimentary metal oxide semiconductor (CMOS) image sensor (CIS) needs to condense more light in a light-receiving portion such as a photodiode. A microlens formed using a photoresist (PR) is typically used to condense light in a photodiode. However, since the PR generally has a low hardness, particles generated during a subsequent wafer-sawing process are often lodged into the surface of the PR, causing damage to the microlens.
To prevent damage to the microlens, an undoped silicate glass (USG) is often deposited on the microlens at a low temperature (typically about 180° C.). The deposited material can also be referred to as a low temperature oxide (LTO).
However, the USG, though hard, can be easily cracked. A crack can affect the microlens formed under the USG, making it difficult to obtain a good image.
In addition, the USG typically has a refractive index ranging from 1.48 to 1.53, and incident light is refracted by the USG when reaching the microlens. As a result, the focal length is reduced, meaning that the thickness at a back-end-of-line (BEOL) operation should be reduced.
Thus, there exists a need in the art for an improved image sensor and fabricating method thereof.